The present invention combines known techniques for stimulating oil recovery in a manner such that viscous oil recovery is increased and made economical. More particularly, viscous oil is recovered from a subterranean formation by means of downhole burning of fuel and hot fluid generation, passing the steam and products of combustion directly into the formation and creating line drive between horizontal drainholes.
In the recovery of oil from viscous oil-bearing formations, it is usually possible to produce only a very small portion of the original in place oil by natural or primary production. A wide variety of artificial recovery techniques, therefore, have been suggested for increasing oil recovery from these formations. For example, it has been proposed to inject gases into the formation to maintain the pressure of the formation and to decrease in a slight manner the viscosity of viscous oil adjacent the gas.
It has also been proposed to use steam stimulation for increasing oil production. Steam stimulation is the addition of thermal energy to a subsurface reservoir by injection of steam, usually of 60 to 90 percent quality. Steam injection has been practiced primarily as steam drive and as steam huff and puff. These techniques have been carried out with and without foaming, surfactant and caustic agents. In the steam huff and puff process, steam is injected into the formation for a period of time. Thereafter, injection is ceased and the well is backflowed, usually by pumping, to produce fluids from the formation. In one variation, the injection step is followed by a period of shut-in prior to producing fluids from the formation. This variation is called steam soaking. In general, steam is injected into the subsurface formation in quantities sufficient to heat a predetermined distance of the formation radially from the wellbore. This distance changes with time and with the number of huff and puff steps performed. Pressures commonly range between 500 and 2500 psi dependent upon the depth of the formation and the permeability of the formation. The steam is injected at a predetermined rate usually stated in pounds per hour or barrels per day of cold water equivalent, and may be injected for periods of a few days to six months and longer depending on the stage of production. In steam huff and puff (including steam soak), the total volume of reservoir heated is not large. Huff and puff through vertical wells is inefficient in formations of very low mobility and permeability. Huff and puff techniques through vertical wells are significantly limited and in many formations are not adequate for producing much of the oil in place.
It has been proposed to inject steam to drive oil toward a vertical production well. The steam displacing fluid travels from the injection well to the production well. The oil within the steam path is the only part of the oil that is produced. The shape of the path of the steam is called the sweep pattern. For vertical wells, at best the steam travels from the injection point to the production point. The steam sweeps a curvilinear path that resembles a pointed ellipse. The volume of the swept area divided by the total volume is called the sweep efficiency. Practical operations require good sweep sweep efficiency. Sweep efficiency or the height and minor axis of the pointed elliptically shaped sweep pattern is dependent on a number of factors. Two factors are the mobility of the oil and the relative mobilities of the oil and steam. Other factors are the distance between the injection well and the producing well and the nature of the wells. The oil or petroleum in most relatively shallow viscous oil bearing formations is immobile until heated. The relative mobilities, densities and other factors cause the steam to override the oil, thereby decreasing the height of the the curvilinear sweep pattern. The mobility of the oil is so low that the minor axis of the pointed elliptically shaped steam path is small and the sweep pattern looks like a sliver or thickened line between the injection point and production point. Premature steam break through occurs and the steam does not lose its heat to the inplace oil. Moreover, the sweep efficiency is very poor. To improve sweep efficiency it has been proposed to drill a line of vertical injection wells and a laterally spaced line of vertical production wells and use a form of displacement called line drive. For line drive between vertical wells the number and costs of the wells required to accomplish the effects of line drive are excessive in viscous oil bearing formations.
In U.S. Pat. No. 3,960,214, it has been proposed to drill a horizontal drainhole in a viscous oil-bearing formation and to drill vertical production wells above and along the length of the horizontal injection well. This process is inefficient. The mobilized viscous oil tends to drain toward the lower part of the formation and the cost of the vertical production wells is excessive. The steam tends to channel through the oil and the sweep effiency is low.
The steam for steam stimulation is usually generated at the surface of the earth and injected through an injection well from the surface. In U.S. Pat. No. 3,456,721, it has been proposed to use a downhole burner apparatus for generating the steam. A fuel is burned downhole with air and water is injected to contact and heat exchange with the flame to turn water into steam. Downhole steam generation has several advantages. The hot exhaust gases are coinjected with the steam into the reservoir as opposed to conventional steam generators which exhaust to the atmosphere. Additionally, the exhaust gases maintain the formation pressure and add a certain amount of viscosity reduction to the oil. Adverse environmental effects on air quality are also reduced.
Some of the more extensive and valuable viscous oilbearing formations are found in areas covered by permafrost. Use of heat through an injection well creates problems in permafrost areas by thawing the area around the injection well.
It is an object of this invention to combine such known techniques in a manner such that many of the above-cited defficiencies associated with the known techniques and vertical wells are avoided or reduced. It is a further object of this invention to provide a viscous oil recovery process that is particularly useful and economical for remote areas, especially the remote areas of Alaska and similar areas where there is permafrost. It is still another object of the invention to provide a process wherein oil is economically produced during the first stage of the process.